1. Field of the Invention
The present invention relates to a semiconductor device, particularly to a semiconductor device that is provided with a protection circuit so as to be protected from destruction due to application of a high voltage such as static electricity.
2. Description of the Prior Art
When a semiconductor device is brought into contact, for example, with a person or machine that is charged with static electricity having a voltage from tens of volts to tens of kilovolts resulting from friction or other, the electrostatic charge is discharged through the semiconductor device via its terminals and internal circuit in a time as short as several nanoseconds to several microseconds. This often causes the so-called electrostatic destruction, by which the internal elements of the semiconductor device are destroyed, or its functions and characteristics are degraded. Especially, in a CMOS, bi-MOS, or similar type of semiconductor device containing field-effect transistors as its circuit elements, the gate oxide films have relatively low breakdown voltages, and accordingly are easily destroyed by application of an extremely high voltage. For this reason, a semiconductor device is usually provided, at each of its input and output portions, with a protection circuit that protects the semiconductor device from destruction by directing the current arising from a high electrostatic voltage to a low-impedance line such as a power source voltage line (V.sub.DD) or reference potential line (GND).
FIG. 1 shows a conventional semiconductor device 10a. This semiconductor device 10a consists of an input/output portion 1a connected to a terminal T1, and an internal circuit 3 that performs signal processing or other. The input/output portion 1a consists of an input/output circuit 2 for buffering a signal between the terminal T1 and the internal circuit 3, and a protection circuit 4a for preventing an excessively high voltage applied to the terminal T1 from reaching the input/output circuit 2 or internal circuit 3. The protection circuit 4a consists of resistors 11 and 14 connected in series between the terminal T1 and the input/output circuit 2, and diodes 12 and 13 connected in the reverse-biased state from the node between the resistors 11 and 14 to the power source voltage line and to the reference potential line, respectively.
The protection circuit operates as follows. Assume that the power source voltage is V.sub.H (V), the reference potential is V.sub.L (V), and the forward voltage of each diode is V.sub.F (V). When a voltage, due to static electricity or other, exceeding (V.sub.H +V.sub.F) is applied to the terminal T1, the resulting current flows through the resistor 11 and the diode 12 along the path indicated by the broken line I1. Therefore, theoretically, the voltage is lowered from the applied voltage by the register 11, and only the remaining portion, i.e. (V.sub.H +V.sub.F), is applied to the input/output circuit 2 and the internal circuit 3, which are thus protected from destruction due to application of a high voltage. Similarly, when a negative voltage lower than (V.sub.L -V.sub.F) is applied to the terminal T1, the resulting current flows through the resistor 11 and the diode 13 along the path indicated by the broken line I2. Therefore, theoretically, the voltage is lowered from the applied voltage by the resistor 11, and only the remaining portion, i.e. (V.sub.L -V.sub.F), is applied to the input/output circuit 2 and the internal circuit 3, which are thus protected from destruction due to application of a high voltage.
However, such a current that flows in through the diodes may cause variations in the voltages of the power source voltage line and the reference potential line. This is because the power lines such as these lines possess various resistances, for example, the resistance of the wiring itself that is formed of aluminum or other metal and the resistance of the contacts between the wiring and the circuit elements.
Especially, when the input/output portion 1a is located away from the power source voltage line and the reference potential line of the semiconductor device, the wiring resistance of the power lines is accordingly greater. For this reason, even if the current is passed only through either the power source voltage line or reference potential line, it is impossible to sufficiently suppress the variations in the voltages of the power lines. As a result, it often happens that the voltage of the power source voltage line becomes higher than (V.sub.H +V.sub.F) or the voltage of the reference potential line becomes lower than (V.sub.L -V.sub.F).
Moreover, in a semiconductor device formed under common manufacturing conditions, the reverse-biased diodes used in the protection circuit and the input gates such as are used in the inverter circuit 2a generally do not have very high breakdown voltages. For this reason, these elements are easily destroyed by application of a high voltage, and, in addition, the diodes are susceptible to thermal breakdown due to a flow of a large current. As a result, when the voltage between the power lines becomes higher than the breakdown voltage of the input/output circuit 1a or internal circuit 3 as a consequence of the voltage variations as described above, it often happens that the circuit elements of the semiconductor device are destroyed, and thus the semiconductor device fails to provide intended functions.
Therefore, in order to protect the semiconductor device from destruction due to the variations in the voltages of the power lines as a consequence of a current resulting from static electricity or other applied to the terminal T1, it is necessary that the resistor 11 have as high a resistance as possible, e.g. over several hundreds of ohms, and that the wiring width for the resistor 11 be widened to allow more current to flow through the resistor 11, so that a sufficient voltage drop and a current-limiting effect are achieved there Moreover, to accomplish securer protection, it is also advisable that the resistor 14 have as high a resistance as possible. This resistor 14 forms an integrator circuit together with the parasitic capacitance of the input/output circuit 2. However, higher resistances in these resistors lead not only to larger time constants, which makes transmission of high-speed input signals difficult, but also to larger chip sizes and higher costs of the semiconductor device.
Furthermore, even static electricity applied from outside a portable electronic appliance may cause destruction of the input/output portion 1a or internal circuit 3 of the semiconductor device 10a mounted on a circuit board inside the appliance, leading to failure of the appliance as a whole, if the semiconductor device 10a is not given a sufficiently high breakdown voltage. In such a case, troubleshooting and remounting of the semiconductor device usually require much time and costs.
Moreover, in a conventional semiconductor device 10a, when the terminal T1 is connected to a bus line or the like connecting to another semiconductor device (not shown in the figure) to which the power source voltage is applied, even if the power source voltage is not applied to the semiconductor device 10a itself, a voltage lower than the input voltage by the forward voltage of the diode 12 is applied through the diode 12 to the power source voltage line of the device 10a. This results in unnecessary power dissipation in the semiconductor device 10a and makes its operation unstable, thereby affecting other semiconductor devices adversely via the bus line or the like.
This problem may be solved by removing the diode 12. However, simply removing the diode 12 results in the loss of the protection capability against electrostatic destruction. Thus, when an excessively high voltage due to static electricity or other is applied to the semiconductor device 10a, its circuits elements may be destroyed and cause faulty operation.